The present invention relates to a process for producing both steam power and cement clinker simultaneously in one apparatus, to the apparatus composed mainly of a coal-powder-burning boiler system which is especially designed to embody said process, to the cement clinker obtainable by said process and also to the use of this cement clinker, the present invention further relates to a process for producing both steam power and a fast-burnt modified coal ash simultaneously while the coal powder is burning in the furnace, to the fast-burnt modified coal ash obtainable by this process, to the apparatus composed mainly of a existing coal-powder-burning boiler system for carrying out this process, to the use of this modified coal ash as cement clinker, and to the use of the existing coal-powder-burning boiler system in production of the fast-burnt modified coal ash, the present invention still further relates to a process for desulfurization from the flue gas of the coal-powder-burning boiler.
Cement is one of the three fundamental materials of the construction industry, it is widely used in a great amount. The most commonly used is portland series cement, GB 175-92 is referred to as its chinese national standard.
The principal raw material for the production of cement clinker is lime stone, clay and iron powder, the fuel used is mainly coal. The active component of the cement clinker is mineral composition comprised mainly of calcium silicates which can be formed usually at 1450xc2x0 C., or at 1300xc2x0 C. while mineralizing agent is added thereinto. The main segments for cement production can, whatever a process and a kiln is used, be summarized as xe2x80x9ctwo pulverizing and one burningxe2x80x9d, namely, the three basic procedures: raw material blending and pulverizing; clinker burning; and cement pulverizing. FIG. 1 is a layout of the rotary kiln technological flow chart for an advanced out-of-kiln predecomposition dry process wherein the most critical apparatus is the clinker burning rotary kiln. In the clinker burning procedure, fuel and raw material are respectively fed into the kiln; the coal powder is injected into the burning kiln through a burner at the kiln end of clinker discharge, a flame like a head of Chinese writing brush is formed at the position of the central line with its high temperature flame up to 1600xc2x0 C.-1700xc2x0 C., heat is transferred by means of convection and radiation to the materials advancing in stacks as the kiln rotates and having a filling rate of about 12-14% in the rotary kiln. There exist two main problems for such heat transfer: one is its low heat efficiency, generally only about 22%, even the advanced out-of-kiln predecomposition rotary kiln dry process has its heat efficiency but up to about 50%; the other is its inhomogeneous heat transfer because the materials is in stacks, causing a longer period of time for the procedures from entry of raw material in the kiln to formation of clinker as well as its cooling (generally it needs about 1.5-2 hour, the out-of-kiln decomposition kiln also needs 40 minutes to 1 hour). The characters of this technology can be summarized as xe2x80x9cslow burning and slow coolingxe2x80x9d; additionally, the clinker is also likely to be over-burnt. The existance of very high burning temperature, inhomogeneous burning and over burning cause a negative influence on cementitious mineral activity and finally influences the performances of the cement clinker. In addition to the above-indicated defects such as high energy consumption, inhomogeneous heat transfer affecting negatively its product quality because the heat transfer is not well, the cement production further brings forth other problems such as consuming huge amount of mine resources, occupying much land, spending large amount of materials transportation coast and resulting in environmental pollution as of land, water source, atmosphere etc.
Coal-powder-burning boiler system is an apparatus widely used for producing steam power, high temperature and high pressure steam produced can be used, for example, for electrical power generation or other purpose. Its principal apparatus flowchart is shown in FIG. 2. It can be seen from FIG. 2 that the coal powder burning boiler system is mainly composed of the following subsystems: Raw coal blending system (including 1,2,3), coal pulverization system (including 4,5,6), coal powder convey and combustion and heat supply system (including 7,8,9,10), and by-product treatment system (including 11,12,13). Among them, the main device is the coal-powder-burning boiler (9). The coal powder, after being injected by a burner into the furnace chamber, is in a form of highly disperse and fludized suspension; while contacting with and being heated by the gas flow of high temperature in range of 1300-1600xc2x0 C. in the furnace combustion zone, the said coal powder burns and release heat; the high temperature and high pressure steam produced in the boiler can be directly used for driving turbine and/or other power or thermal engineering apparatus. The residence time of coal powder or its ash substances in the high temperature zone of furnace chamber averages generally about 2-6 seconds with the possibly highest reaching 8 seconds. The coal ash formed exits the furnace chamber along with the flue gas flow at the temperature of about 1200xc2x0 C. and enters the dust-containing flue gas duct, and carries out indirect heat exchange with the steam, feed water and blowing-in air of the boiler, during this procedure, the formed coal ash cools down quickly and homogeneously, and then enters in the electrical dust-remover (11) and is collected therein. Meanwhile, a small amount of slag fall down to the bottom of furnace and is quenched by water and then collected and discharged. The characters of this process, i.e. the fast burning and heat releasing in the furnace, and the fast formation and cooling down of the coal ash and slag can be summarized as xe2x80x9cfast-burning and fast cooling; the collected coal ash is usually delivered hydraulically into the specially-built ash pits (13) and/or pneumatically into the tanks for storage; after coal ash is removed, the exhaust gas containing a minor amount of coal ash dust and some SO2 is emitted into the atmosphere.
Such a coal-powder-burning boiler has large heat-transfer area and large heatransfer coefficient, so its thermal efficiency can reach 90% or even ligher. However, the operation of such coal-powder-burning boiler brings forth two rather serious problems: one is: a large amount of by-produced coal ash gives rise to rather serious pollution, e.g., a 850 mw thermal power plant will spend each year 20 millions RMB or more and occupies much land for building or extending works of ash piling pits, will consume much water and power for transporting coal ash, and will pollute the environment because the coal ash flies with the wind or leaking along with rain water from the ash piling pits; the other is that the exhaust gas emitted from the chimneys carries rather large amount of SO2 and the residue of fly ash into air which also cause atmospheric pollution.
Since a long term, many approaches have been researched and developed in order to treat and utilize comprehensively the waste ash and slag collected from the coal powder-burning boiler. Among them, the main one is to use it as an admixture i.e., the filler to produce coal ash cement, but the addition of coal ash is generally limited to 20-40% (refer to GB 1344-92), so both the rate and the value thereof are not high; further, the coal ash transporting from the steam powder plant to the cement works needs again large amount of transport fee. One of the main reasons which causes the addition of coal ash being incapable of going beyond 40% is that the coal ash turns out to be a kind of pozzolan, featuring a chemical composition of Si-riched and Ca-lack, it has some potential hydrauric cementitious activity. In order to develop such a potential activity, many measures have been taken, but these measures mostly have been directing to the ready coal ash which had come out of the furnace before treatment, the main problem thereof is that its potential activity is very much limited, correspondingly its activity capable of being developed is also very much limited and is by no means to be comparable to the good hydraulic cementitious activity of the cement clinker, therefore, this problem is not substantially solved.
For example, the xe2x80x9cTest and Research on coal ash hydraulic activity-improving techniquesxe2x80x9d ( less than  less than China Building materials greater than  greater than , May, 1995) discloses a process for activating coal ash, wherein activating agent is added into cool ash after its formation and then conducted calcination at 650-800xc2x0 C. for the modification. Such a process is still limited to Activating the potential activity of the coal ash, and the coal ash activated thereby is still incapable of being used as the main cementitious substance to produce coal ash cement, thus it fails to raise its utility rate and value.
Recently, a process for modifying coal ash by adding lime stone powder into coal powder and then burning in the coal-powder-burning boiler has been developed (xe2x80x9cDevelopment of high calcium content coal ash and its use for cement production Sand saving clinkerxe2x80x9d,  less than  less than Chinese Building Material Sciences. And Technology greater than  greater than , No.5,1993). However, as the decomposition of lime stone into CaO and CO2 is a endothermic reaction, it absorbs a large amount of heat in the boiler furnace chamber, influencing the normal combustion and steam power production of the boiler, or even worse, resulting in a fire-extinction accident of the furnace chamber. So the addition amount of lime stone can only be very limited and the maximal Ca content in the so-calledxe2x80x9d high calcium content (expressed is CaO % by weight, based on the total weight of the coal ash.) coal ashxe2x80x9d obtained according this prior art process is generally about 20%, even though the activity of such modified coal ash is somewhat raised, it is still used together with CrFe ash containing mainly xcex2-dicalcium silicate (xcex2-C2S) and xcex3-dicalcium silicate (xcex3-C2S) to produce masonry cement of low grade below 325# (see CN. 9110608.4). Therefore, its use is rather narrow and fails to essentially satisfy the demaned of the entire and highly-valued use of coal ash. Another difficulty in use of coal ash lies in that the detrimental effect of the residual carbon in the coal ash (or as usually called xe2x80x9closs on ignitionxe2x80x9d) impairs the quality and performances of the resulted coal ash cement product. The residual carbon in the coal ash is porous and possesses a very strong water adsorption, its existence leads to a increase of water demand for the ash and lowers strength of the coal ash agglomerate, resulting in the decreasing of the anti-permeating and anti-frozen performances of the resulted product; at the same time the residual carbon tends to form a hydrophobic membrance on the surface of the product, which hinders the active substance from hydrating reaction, giving a dispersing and destructive effect on aggregation of the coal ash; additionally, the existence of residual carbon also lowers the utilization rate of coal""s thermal value and the boiler""s thermal efficiency.
According to the prior art, steam power plant and the cement works belong to different industrial categories and are respectively operated and administered, they have respectively their own problems.
Based on the knowledge that there exist, in the existing technology of cement clinker production characterized by slow burning/slow cooling, the problems of poor heat transfer and product quality, the present inventors have devoted them selves since long to developing new technologies for cement clinker production. Through long term""s observation and research on the combustion characteristics of various furnaces and kilns, and being enlightened by the thermal engineering conditions of the coal-powder-burning boiler system characterized by xe2x80x9cfast burningxe2x80x9d of the coal powder and xe2x80x9cfast-coolingxe2x80x9d of the coal ash, the inventors present the concept of utilizing such a favourable thermal engineering condition of the coal-powder-burning boiler system to produce cement clinker. In order to embody this concept, three main problems need to be solved. Firstly, how to have the chemical composition of the coal ash obtained from the coal-powder-burning boiler meeting the requirement of the desired cement clinker; secondly, how to make the coal ash substances have the mineralizing reaction during coal combustion, so as to form substantial amount of mineral composition which possesses good hydraulic cementitious activity; thirdly, how to assure that the mineralizing reaction of coal ash substances does not consume any heat energy from the furnace chamber so as to keep the furnace chamber at high temperature condition and to maintain the steam-supply capability of the boiler.
In order to solve the above three problems, the present inventors conduct a deep-going research on the combustion course of the coal powder in the boiler furnace chamber and the physico-chemical properties of the coal ash, they find that the principal chemical components of coal ash in China are four oxides: SiO2, Al2O3, Fe2O3 and CaO, which are just the same as the principal chemical components of the cement clinker (referring to its oxide form), the chemical components and their contents of both the cement clinker and coal ash in China, respectively, are in Table 1.
It can be seen from Table 1 that, though both of them contain same sorts of oxides, yet their content ranges of the components are rather different, the main distinctions that the CaO content in the coal ash is far lower than that of the cement clinker, this lower Ca content is the main cause why the coal ash possesses no or little mineral phases such as dicalcium silicate (C2S), tricalcium silicate (C3S), tricalcium aluminate (C3A) and tetra calcium ferrous aluminate (C4AF) etc. which are contained in cement clinker and endows it with hydraulic cementitious ability. In view of the above-indicated features, the present inventors realize that it is possible to make the chemical composition, mainly the calcium content, of the combustion residue meeting that of the desired cement clinker, if the coal is, based on the content and chemical composition of the coal ash substance contained in raw coal and in compliance with the requirement for achieving the cement clinker""s chemical composition, blended with an admixture containing mainly calcium-rich substance, substantially in form of CaO, and optionally some other additives such as mineralizing agent, crystal seed, early strong agent and loosener etc., hereinafter referred to as xe2x80x9cAMCxe2x80x9d. The inventors further realize that the burning and forming temperature of the cement clinker is about 1450xc2x0 C., or 1300xc2x0 C. when mineralizing agent is added thereinto, while the temperature range 1300-1700xc2x0 C. in the high temperature zone of the furnace chamber of the coal-powder-burning boiler can well satisfy the temperature requirement for burning and forming cement clinker, the above-indicated blended coal mixture is ready to form large amount of cementitious mineral phase when it is burnt at high temperature in the furnace chamber. The present inventors have also conducted a deep-going research on the cause why prior art, particularly the prior process known from the xe2x80x9cDevelopment of high calcium content coal ash and its use for cement production and saving clinker,  less than  less than Chinese Building Material Science And Technology greater than  greater than , No.5,1993.xe2x80x9d fails to effectively modify the coal ash and endow it with good hydraulic cementitious activity by adding lime stone; they find that in the said prior art process the added lime stone CaCO3, while decompose into CaO and CO2 in the furnace chamber, absorbs a rather large amount of heat from the chamber, thus resulting in detriment to the high temperature condition of the furnace chamber and decreasing the steam power supply capability of the boiler, so that the addition amount of CaCO3 can only be very limited and the CaO content in the coal ash finally obtained in this way fails to exceed 20%, still far behind the calcium content of the cement clinker; they also find that the residence time of the coal powder in the high temperature zone of furnace chamber is but totally 2-seconds, while decomposition of CaCO3 into CaO and CO2 needs actually over 2 seconds, thus the formed CaO has not enough time in the chamber to have the mineralizing reaction with the other chemical components, and the coal ash obtained by such process contains by no means significant amount of cementitious mineral phase, still remaining to be a coal ash with low cementitious activity. Thus, the resulted coal ash is not so modified for producing portland coal ash cement of high grade and has no possibility to be directly used as cement clinker in high grade cement production.
Based on the above knowledge, the present inventors discover that the above mentioned two problems existed in the prior art process in which lime stone is added can be completely overcome if AMC is blended with the coal, namely, there exists no problem of the heat absorption of lime stone decomposition in the furnace, on the contrary, as the mineralizing reaction between CaO and coal ash substances is an exothermic reaction, these reactions are favourable for raising the furnace chamber temperature and improving the combustion and are favourable for maintaining or even raising steam power supply, hence the AMC addition amount can be greatly increased, resulting in the CaO content in the residue to reach 20-70% after the combustion of the blended coal mixture; additionally there exists no problem of time consumption for CaCO3 decomposition as the AMC mainly containing calcium-rich substance, substantially in form o CaO, is now blended, the CaO has sufficient time to conduct the mineralizing reaction with coal ash substances during the coal powder combustion in the furnace chamber and to form the mineralized substances with hydraulic cementitious activity.
The present inventors also further find that if the AMC is sufficiently and finely integround with coal, these two sorts of powder will have intimate contact each other when this finely integround powder is injected into the furnace chamber for combustion, they are not only exothermal fuel, but also the reactants as such for the mineralizing reaction during the combustion; it not only suspends and burns in the furnace at a high combustion speed, but also conduct a homogeneous direct-contact heat-transfer and masstransfer, the combustion heat releasing and the mineralizing reaction take place almost simultaneously and quickly finish. The oxides in the coal ash substances such as SiO2, Al2O3 and Fe2O3 exist in the form of kaolin (Al2O3.2SiO2.2H2O), their heat-absorbing dehydrolization and decomposition coincide with the exothermic mineralizing reaction with calcium-containing substances contained in the AMC, assuring the high temperature burning between the AMC and the oxides newly-generated from the decomposition of the coal ash substances and rapid formation of mineral phases such as C2S, C3S, C3A and C4AF etc. which have hydraulic cementitious activity. The calculation results obtained from the thermal balance for the entire decomposition heat absorption during the decomposition and heat release during the mineralizing reaction show that, as a whole, the heat release is larger than the heat absorption, this further guarantees a favourable high temperature condition of the furnace chamber and the boiler""s steam power supply capability. After exiting from the furnace chamber, the combustion residue is quickly cooled and the microcrystals formed in mineralizing reactions have not enough time to develop themselves during this fast cooling course, and some of them appears to be in the vitroclastic microcrystal intermediate phase which possesses large free energy and many structural defects, thus it can further raise the hydraulic cementitious activity of the residue and make it substantially into cement clinker with good hydraulic cementitious activity.
Therefore, one object of the present invention is to provide a novel process for xe2x80x9cproducing both steam power and cement clinker simultaneously in one apparatus. According to this process, the two courses of steam power supplied by coal powder combustion and cement clinker production are combined in one apparatus composed mainly of coal-powder-burning boiler system and are carried out simultaneously without increasing the coal consumption of the boiler system.
Another object of the present invention is to provide a new type of cement clinker which is obtainable according to the inventive process. A most important difference of it from the conventional cement clinker produced by prior art lies in that, under the premise of having basically same chemical composition for the two, the cement clinker of the present invention is obtainable employing the new xe2x80x9cfast-burning and fast coolingxe2x80x9d technology and has higher hydraulic cementitious activity than that of the conventional one. In order to distinguish it from the conventional cement clinker, the cement clinker of the present invention is called xe2x80x9cfast-burnt cement clinkerxe2x80x9d hereafter.
Further another object of the present invention is the cement made of such fast-burnt cement clinker. The fast-burnt cement clinker can be used as the sole cementitious active substance or as main part be mixed with a minor amount of conventional cement clinker to be combining with other conventional cement additives such as gypsum to be interground into cement. In order to reflect the distinctive characeristics of the cement products made solely or mainly of the fast-burnt cement clinker of the present invention, this cement is designated as the xe2x80x9cfast-burnt cementxe2x80x9d here below.
Further another object of the present invention is to provide a set of apparatus for simultaneous production of steam power and the fast-burnt cement clinker and, optionally, preparing the fast-burnt cement from the fast-burnt cement clinker.
Still another object of the present invention is to provide a process for simultaneous production of steam power and direct modification of coal ash during coal powder combustion in the boiler furnace. According to this process, a fast-burnt modified coal ash possessing good hydraulic cementitious activity can be produced by adding AMC into raw coal while assuring the steam power supply without increasing coal consumption.
Still another object of the present invention is to provide the fast-burnt modified coal ash obtainable according to the above-indicated process. This fast-burnt modified coal ash possesses good hydraulic cementitious activity and can be used as the sole cementitious active substnace or as main active substance together with a small amount of conventional cement clinker, for producing portland fast-burnt modified coal ash cement with good performances. So, this fast-burnt modified coal ash is in fact the fast-burnt cement clinker.
Still another object of the present invention is a new kind of cement prepared using this fast-burnt modified coal ash. The fast-burn modified coal ash can be used as the sole cementitious active substance, or as main active substance together with a small amount of conventional cement cliker, and further mixed with some other conventional cement additive and then be ground into portland fast-burnt modified coal ash cement.
Still another object of the present invention is the use of coal-powder-burning boiler system in the production of the fast-burnt modified coal ash.
Still another object of the present invention is to provide a apparatus which can embody the inventive process for simultaneous production of steam power and fast-burnt modified coal ash and, optionally, for making the latter into Portland fast-burnt modified coal ash cement. The existing conventional coal-powder-burning boiler system may be used to construct the xe2x80x9cinventivexe2x80x9d apparatus for simultaneous production of steam power and fast-burnt modified coal ash portand cement, the only additions thereto are: a set of AMC pregrinding and blending system is connected ahead of the coal grinding machine and a set of material blending, pulverizing, packing and/or storage system for coal ash cement products is connected behind the collection of the fast-burnt modified coal ash.
Still further another object of the present invention is to provide a process for combining sulfer contained in the coal into the coal ash and for further removal of the SO2 from the flue gas of the coal-powder-burning boiler.